The present invention relates to an improvement in a coupling circuit for A.C. coupling a switching network in a telephone exchange apparatus to a trunk circuit through a transformer.
FIG. 1 shows a prior art coupling circuit for coupling a switching network in a telephone exchange apparatus to a trunk circuit. In FIG. 1, numeral 1 denotes the switching network, 2 the trunk circuit, 3 a transformer and 4 a remote station.
In the prior art coupling circuit, when the switching network 1 is coupled to the trunk circuit 2 through the transformer 3, it has been a common practice to supply a direct current for holding selected switches in the switching network 1 and to supply a direct current between the other winding, i.e., the secondary winding, of the transformer 3 on the side of the trunk circuit 2 and the remote station 4 so that a voice signal is communicated between the remote station 4 and the trunk circuit 2 in accordance with the polarity (or direction) of the latter current. Thus, the current in the winding of the secondary transformer 3 on the side of the trunk circuit 2 flows in one direction or the other depending on the signal condition and has no constant direction. Accordingly, the transformer 3 must be designed to take into consideration the possibility that a condition might occur wherein direct currents flow in the respective windings of the transformer 3 in the same direction such that nonvarying magnetic fluxes produced by the D.C. component of the transformer current on the side of the switching network 1 and that on the side of the trunk circuit 2 are additive in the core of the transformer in which a magnetic flux path is established. In such case, a large direct current is superimposed on the transformer, resulting in the need to increase the size of the transformer to avoid saturation thereof, which will produce deterioration of the A.C. transmission characteristic through the transformer.